Signaling circuit



Dec. 28, 1937. H. o. PETERSON ET AL 2,103,869,

SIGNALING CIRCUIT Filed 001:. 26, 1933 4 Sheets-Sheet 1 38v I v ac SOURCE 1 b Q, Q S Q Q $5 INVENTORS MARTIN KATZIN H.O. E flfOWOflf Pom/7m BY 9 TERSON ATTORNEY Dec. 28, 1937. o PETERSQN ET AL 2,103,869

SIGNALING CIRCUIT Filed Oct. 26, 1933 4 Sheets-Sheet 2 2a 6'0 a0 50 m0 #0 7% 92 INVENTORS MARTIN KATZIN BY fZOjETERSQN ATTORNEY Dec, 28, 1937. o PETERSON ET At 2,103,869

SIGNALING CIRCUIT Filed Oct. 26, 1953 4 Sheets-Sheet 5 INVENTORS MARTIN KATZIN H. O. PETERSON ET AL SIGNALING CIRCUIT Dec. 28, 1937.

Fiied Oct. 26, 1933 4 Sheets-Sheet 4 Huh.

, ATTORNEY Patented Dec. 28, 19 37 PATENT OFFICE SIGNALING CIRCUIT HaroldOlaf Peterson and Martin Katzin, Riverhead, N. Y., assignors to Radio Corporation of America, a corporation of Delaware Application October 26, 1933, Serial No. 6955254 18 Claims.

This invention relates to the art of signaling and more particularly to the art of modulation and demodulation of carrier oscillations by means of thermionic tubes, and more specifically 5 to the methods for modulating and demodulating carrier frequencies which employ multi-grid tubes.

This invention makes possible substantially complete modulation of a carrier wave without appreciable distortion andyet at the same time requiring a relatively small amount of modulation power. The invention also makes possible efiicient and complete linear demodulation of a signal modulated carrier wave. In this demodulation spurious responses known as harmonic points are eliminated. The method of demodulation is particularly applicable to receivers of the heterodyne type. Modulation systems which have heretofore been employed have been unable to completely modulate the carrier wave without considerable distortion and/or have required a large amount of modulating power to accomplish modulation of the carrier.

Demodulation systems of the heterodyne type known heretofore in some cases do not produce linear demodulation and such systems may respond to interfering signals as well as the desired signal if the interfering signal is at a frequency separated from the local frequency by a difference frequency equal to the beat frequency. Or the desired signal may appear at two different settings of the tuning elements.

Briefly, an object of this invention is to accomplish distortionless modulation of a carrier frequency by employing thermionic tubes having a multiplicity of grids or controlling electrodes, applying the voltage of the carrier frequency to one of the grids and'the voltage of modulation frequency to two or more grids in-such relative amounts and phases as to result in substantially distortionless modulationfor all degrees or depths extending to substantially complete modulation.

' In a modification'the voltage of the modulating frequency may be applied to two or more grids and 'to the anode in such relative amounts and phases as to result in substantially distortionlessmodulation for all degrees or depths extending to substantially complete modulation.

Another object of the present invention is to accomplish linear and complete demodulation of a signal modulated carrier frequency by employing thermionic tubes having a multiplicity of control grids or electrodes, applying the signal modulated carrier "frequency oscillations to one of the grids and the local oscillations to two or more grids in such relative amounts and phases as to result in substantially linear demodulation by the heterodyne method free of interfering signals and image frequencies.

In a modification the signal carrying modulated wave may be applied to two or more of the grids in certain relative amounts and phases, while the local oscillations may be applied to a single grid. 7

This invention makes possible substantially complete modulation of the carrier without appreciable distortion, yet at the same time requiring a relatively small amount of modulation 7 power. 7

Improved demodulation may also be obtained by utilizing the principles of the present invention. v

The invention has, as required by law, been pointed out with particularity in the claims appended hereto.

The method and mode of operation of the same will be best understood from the specification and therefrom when read in connection with theattaohed drawings, throughout which like reference characters indicate like parts, and in which:

Figures 1 and la show schematically a modulator including features of the present invention;

Figures 2a, and 2b are a wiring diagram showing the application of practical potentials and curves derived by operation of the tube of 211 under certain conditions; a I

Figure 3 is a characteristic curve illustrating results obtained under certain conditions;

Figures 4a., 4b and 5 are modifications of the arrangement of Figure 1;

' Figure 6 is a circuit illustrating the application of the principles of the present invention to demodulation;

Figure 7 is a modification of the arrangement of Figure 6;

Figure 8 illustrates the manner in whiohthe principles of the present invention may be applied 'to demodulation circuits of the nonheterodyne type; while,

Figure 8a is a curve illustrating the operation of the device of Figure 8.

The present invention may best be described by reference to the drawings of Figures 1, 2a and 2b.

Figure 1 shows one specific embodiment of the invention in which a complete telephone transmitter is represented. Referring to Figure 1, 4 is a thermionic tube which, with the associated tuned circuit formed by inductance 9 and capacitance 8, together with its associated equipment, form a conventional oscillator circuit, well known to the art, for the generation of wave energy of carrier frequency. Coupling coil l3 serves to transfer energy from the oscillatory circuit to the innermost grid 29 of the threegrid thermionic tube 2|, which grid is biased to a suitable negative potential with respect to cathode IE by means of battery M. The grids l8 and |9 are by-passed for the radio frequency by means of condenser 22 and choke coil 25, and condenser 23 and choke coil 24, respectively. Grid I8 is maintained at suitable positive direct current potential by means of battery 32, while grid I9 is shown as being maintained at suitable negative potential by means of battery39. Grid It! may be the usual suppressor grid, E8 the usual screen grid, and 29 the usual control grid of a tube. Voltage variations of modulation frequency, generated by the microphone 3|, in series with a source 3!], are applied to the two grids by means of transformer 28. Condenser 26 and choke coil 21 serve to isolate grid l9 from the battery 32 of grid l8, yet to allow the modulation voltages to vary the potential of grid It. Plate 20 is connected to inductance 34 and condenser 35, which together tune to the carrier frequency and form the output circuit across which the modulated wave appears. Coupled to inductance 34 is inductance 3B, which transfers the modulated Wave to antenna 38, which radiates the waves into space.

The operation of the modulator tube 2| in Figure 1 may best be described with reference to Figures 2a and 2b.

The tube of Figure 2a represents the tube 2| of Figure 1 and shows the connection of the various voltages to the several electrodes of the tube. The curves of Figure 2b represent the transconductance between grid G1 and plate P for various operating potentials on the several electrodes. Since the output voltage produced across the load in the plate circuit of the tube is proportional to the transconductance between the input electrode and the plate, the curves of Figure 2b also represent the voltage output obtainable under the several conditions.

Curve A of Figure 2b shows the variation of transconductance with Eg2, the voltage between grid G2 and cathode K, showing that considerable distortion would be entailed by applying'modulating voltage to grid G2. Similarly curve B, showing transconductance versus Eg3, the voltage between grid G3 and cathode K, shows that high modulation percentages would be subject to considerable distortion.

Curve C shows the improvement obtained by varying the potentials of G2 and G3 simultaneously by equal amounts. The result is that the curved portion near the cut-off point is reduced and the cut-off made sharper. The reason for this is that as G3 is made more negative, G2 is simultaneously made less positive, and thus assists in the reduction of transconductance. For composite modulation corresponding to curve C,

operation at the direct current electrode potentials corresponding to point a on the curve would be most suitable. As a result, the composite curve C would allow higher modulation percentages without distortion than would curve B.

Although, for purposes of illustration we have shown several sources of potential for energizing the electrodes of the oscillator and modulator in Figure 1, it will be understood that in practice a single source of direct current potential may be connected, as shown in Figure 1a,, with a resistance R. in such a manner that all of the direct current potentials necessary to energize the circuit may be derived from points on this resistance R. The several direct current leads from the electrodes in the tubes may be connected as indicated to points on the potentiometer. Moreover, in all of the modifications described hereinafter including the demodulating circuits, it will be understood that the several batteries utilized in energizing the several electrodes of the several tubes may be replaced by a single'source of potential connected as indicated in Figure 1a.

The operation of this device is not restricted to simultaneous variation of the two grid potentials in equal amounts, as described in the specific embodiment above. The basic principle is to vary simultaneously the potentials of two or more electrodes in the proper proportions to obtain linear operation and therefore minimum distortion. One example is shown by curve D of Figure 2b, in which the potential of grid G2 is changed by twice the amount as G3. Operation at the direct current potentials corresponding to point b of this curve would allow substantially complete modulation to be obtained with very little distortion.

In some cases a characteristic is obtained in which the upper portion curves upward, as in Figure 3. In such a case, it usually happens that as the electrode being modulated becomes increasingly positive, or less negative, a simultaneous reduction in voltage of some other electrode will have the effect of counteracting the upward swing of the curve and therefore prolong its linearity. This case can be treated by applying modulating voltage in opposite phase to this second electrode, as, for example to electrode 20 in Figure 41), or often by inserting a resistance in series with the anode electrode supply circuit, such as resistance 42 in Figure 4a.

In Figure 4a the modulating potentials are applied in phase to the grids l8 and i9 as in Figure 1. The resistance 42, inserted in the anode circuit, however, opposes changes in anode current and in this manner prolongs the straight portion of the curve.

In Figure 4b the modulating potentials are applied, as shown, from microphone 3| to the primary winding of a transformer 28 which has several secondaries which apply the modulating potentials in phase to the grids I8 and I9 and in phase opposition to the plate 2%.

In the modifications illustrated in Figures 1, 4a, and 4b the grids l8 and i 9 are moved (potentially) by the modulating potentials in the same direction. In Figures 4a and 4b only the plate moves or swings in the opposite: direction with'respect to the two grids.

The upper end of curves C and D in Figure 2b, which remain curved, but are not utilized as part of the modulation characteristic by virtue of the selection of the direct current potentials of the electrodes to correspond to points a and b, respectively, may be further straightened by connecting a negative resistance device in series with electrode G2, so that as the current drawn by this electrode increases due to a positive excursion of the modulating voltage, the voltage applied between electrode G2 and cathode K will be still further augmented by the negative resistance device, thus still further increasing the transconductance. This would allow moving points a and b higher upon the curve and thus give greater modulated output from the device.

, a stage of radio frequency amplification.

Although the specific-embodiment of the modulation circuit described above is to be used with a three-grid, tube, it is understood that the principles of this invention are applicable to a tube of any number of electrodes in which linear modulation is obtained by simultaneous variation of the potentials of twoor more electrodes at the modulation frequency. For example, a Circuit arrangement as shown in Figure 5 may be utilized.

In Figure 5 a thermionic tube 2| includes four grid electrodes 29, |8', l9, and I9 and an anode 20 connected as shown. Grid 29 is supplied with oscillatory energy by means of a coil I3 coupled as shown -to any oscillatory energy source, as, for example, an oscillatory tank circuit. Grids l8 and IS, in addition to their respective normal biasing potentials derived from sources 32 and 32, are varied in potential cophasally at an audio frequency rate by transformer 28 connected .as shown with source 3|. A resistance R and a source 39 are incorporated in the circuit of grid l9 so that as the current of grid l9 increases, as the result of an excursion of the voltages of grids l8 and IS in a positive direction, the voltage at I9 will decrease. The various operating conditions andcircuit constants are so chosen that the resulting modulation characteristics are closely linear. The anode circuit, bypassing condenser 40, etc., may be. connected, as shown, and as shown in the prior modifications.

Here, then, we swing the grids l8 and |9- in phase while the grid I9 is caused to move in phase opposition. The plate potential may remain comparatively fixed with respect to the modulation frequency 'or may bev swung at the audio frequency in the manner to assist the various effects to form the characteristic desired.

Under certain conditions the desired linearity of modulation may be obtained by the use of a three-grid circuit, as shown in Figures 1, 4a, and 4b, modified to the extent necessary to swing the grids in phase opposition. This might merely require selection of the inductive and/r capacitive elements 24, 25, 26 of Figures 1 and 4a, and of said elements and the transformer windings S1, S2 and S3 of Figure 4b.

The manner in which the principles of the present invention are applied to demodulating circuits of the heterodyne type will be described in connection with Figure 6 which is intended merely for purposes of illustration and not by way of limitation of the invention.

In Figure 6 tube A and associated circuits form The anode circuit of tube A may be connected with a tunable circuit TC including an inductance- -coupled with an inductance 3 connected with the control grid 29 of a tube 2|, which in this case acts as a demodulator tube. O is an oscillator including a thermionic tube T having output and input velectrodes coupled, as shown, to

an oscillation producing circuit including fre- V quency determining means. The oscillation producing circuit includes an inductance I coupled, as shown, to an additional inductance I. The inductance I supplies energy from the heterodyne oscillator O by way of inductance I to the grids l9 and IQ of tube 2|. In this case the grid It serves as a shield between the signal circuits and the local heterodyne oscillator O. The demodulated signals appear in the anode circuit including the choke 4| and capacity 40 and may be impressed on any utilization circuit by way of an intermediate frequency amplifier.

. The use of two grids fed with energyfrom the heterodyning frequency oscillator provides a linear operating characteristic as described in the original disclosure. This results in improved demodulation characteristics in the elimination of. spurious responses known as harmonic points. A brief explanation of the origin of these spurious responses will show how the use of the multi-grid modulation principle helps to eliminate them.

Suppose we have a superheterodyne receiver whose intermediatefrequency amplifier operates at.a frequency of 100 kilocycles. Let us tune this receiver to receive a signal S whose frequency is 10,000 kilocycles. 'I'he heterodyne oscillator will therefore be set at a frequency B of either 9900 kilocycles or10,100; let us choose 10,100 kilocycles. If, in addition to the desired signal S, there is also present a signal (undesired) whose frequency I is 10,200 kilocycles, this second signal will also beat with the heterodyne oscillator frequency to form a frequency of 100 kilocycles. This undesired signal I is called the image frequency and it is primarily for the purpose of eliminating this image that radio frequency amplification (and selectivity) is added ahead of the heterodyne demodulator.

Now suppose that, in addition to the desired signal S, the input'to the receiver contains a frequency H of 10,050 kilocycles, separated from the heterodyning frequency B by half the in-. termediate frequency. If the signal grid characteristic is not perfectly'linear, that is, if the signal'grid to platetransconductance versus signal grid voltage is not a straight line, and similarly if the heterodyne frequency grid characteristic is not linear, then there will appear a number of frequencies in the plate circuit among which is one, 2 (H-B), which equals 100 kilocycles: Therefore, the frequency H (10,050 kilocycles) will give rise to interference. Similarly, other interferences will be encountered for frequencies separated from the heterodyning frequency, by integral fractions of the intermediate frequency, i. e., /2, A, /5, etc., of the intermediate frequency. These interferences or spurious responses are known as harmonic point responses.

Now if either the signal grid characteristic or the heterodyning grid characteristic is absolutely linear, then these harmonic point responses will not appear. It is to this end that the principle of double grid modulation would prove useful in a superheterodyne receiver. Obviously, either characteristic could be made very nearly linear by using the two grid principle, so that be applied to each frequency so that we would have a five grid demodulator, the middle grid acting as a shield between the two'circuits. We havenot shown a diagram of this connection, but the extension is obvious.

While the invention has been illustrated as being applicable to heterodyne demodulators in which a four-grid tube is preferably used so that a screening grid may be interposed between the signal modulation carrier receiving circpits and 7 V the heterodyne circuits, it will be understood that the invention is also applicable to non-heterodyne demodulation. In such a case a three-grid tube may be used. A signal modulated carrier may be applied to two grids while the inner grid is suitably biased. The biases on the first grids are such that the tube is in the cut-off position when no signal is applied. A circuit of the nonheterodyne type, as, for example, a second detector in a heterodyne circuit, has been shown for purposes of illustration in Figure 8.

While we have shown a three-grid tube in this modification, Figure 8, it is understood that grid 29 and its associated biasing battery may be omitted, thus allowing the principles of this modification to be effected with a two-grid tube.

In Figure 8 the radio frequency amplifier A is coupled by way of its output circuit TC to an inductance l3 which impresses the signal modulated carrier frequency oscillations amplified by A in the desired phase relationship upon the two outer grids l9 and 19'. The inner grid 29 is in this case biased to a suitable potential by a source as shown. The direct current potentials for the grids l9 and I9 respectively are fixed by the direct current sources 39 and 32' connected as shown.

The potential of the sources is so selectedthat the tube is operated in the cut-off position when no signal is applied. This point is indicated at a in Figure 8a of the drawings.

The linear demodulated oscillations may be fed from the anode circuit including the transformer T2 to a work circuit directly or by way of additional amplifiers including one or more thermionic tubes 50 connected with the secondary winding of transformer T2.

Having thus described our invention, what we claim is:

1. The method of effecting distortionless modulation of carrier frequency oscillations by means of a thermionic tube having a control grid electrode, a cathode electrode, an anode electrode and a plurality of intermediate auxiliary electrodes which includes the steps of, impressing the carrier frequencyoscillations on said control grid electrode, and applying the modulating potentials in phase to a pair of said auxiliary electrodes while maintaining the same at different direct current potentials with respect to said cathode.

2. The method of superimposing two oscillating potentials to obtain resultant energy truly characteristic of one of said potentials by means of a thermionic tube having a plurality of grid electrodes and an anode which includes the steps of, impressing one of said oscillating potentials on one of said grid electrodes, impressing the other of saidoscillating potentials in phase on a diiferent pair of said grid electrodes, and deriving said resultant energy from said anode electrode.

3. The method of effecting distortionless modulation of carrier frequency oscillations by means of an electron discharge tube having a cathode, a plurality of grid electrodes and an anode which includes the steps of, impressing the carrier frequency oscillations on one of said grid electrodes, applying the modulating potentials to a pair of said grid electrodes, maintaining one of said grid electrodes at a positive direct current potential relative to the cathode of said tube, maintaining the other of said grid electrodes at a negative direct current potential relative to. the cathode of said tube, and deriving the modulated energy from said anode.

4. The method of effecting distortionless modulation of carrier frequency oscillations by means of a thermionic tube having a control grid electrode, a cathode electrode, an anode electrode, and a plurality of auxiliary electrodes which includes the steps of impressing the carrier frequency oscillations on said control grid electrode, and applying the modulating potentials in phase to a pair of said auxiliary electrodes and in phase opposition to said anode while maintaining said electrodes at different direct current potentials with respect to said cathode.

5. The method of effecting distortionless modulation of carrier frequency oscillations by means of a thermionic tube having a cathode electrode, an anode electrode, and a plurality of grid electrodes which includes the steps of impressing the carrier frequency oscillations on one of said grid electrodes, and applying the modulating potentials in phase to a pair of said grid electrodes and in phase opposition to said anode.

6. The method of signaling by means of a thermionic tube having a cathode electrode, an anode electrode, and a plurality of auxiliary electrodes which includes the steps of, impressing oscillations on one of said auxiliary electrodes, and applying oscillations of different frequency to a pair of said auxiliary electrodes and to said anode while maintaining said electrodes at different direct current potentials with respect to said cathode.

7. The method of effecting distortionless modulation of carrier frequency oscillations by means of a thermionic tube having a control grid electrode, a cathode electrode and a plurality of auxiliary electrodes which includes the steps of, impressing the carrier frequency oscillations on said control grid electrode, and applying the modulating potentials in phase to a pair of said auxiliary electrodes while maintaining the same at different direct current potentials with respect to said cathode, one of said auxiliary electrodes being maintained at a potential equal to the potential of the other auxiliary electrode plus a constant.

8. The method of signaling by means of an electron discharge tube having a cathode electrode and a plurality of auxiliary electrodes which includes the steps of, impressing oscillations on one of said auxiliary electrodes, and applying oscillations of different frequency substantially in phase to a pair of said auxiliary electrodes while maintaining the same at different direct current potentials with respect to said cathode, one of said auxiliary electrodes being maintained at a potential equal to the potential of the other auxiliary electrode plus a constant.

9. The method of eifecting distortionless modulation of carrier frequency oscillations by means of a thermionic tube having a control grid electrode, a cathode electrode and a plurality of auxiliary electrodes whichincludes the steps of, impressing the carrier frequency oscillations on said control grid electrode, and applying the modulating potentialsin phase toa pair of said auxiliary electrodes while maintaining the same at different direct current potentials with respect to said cathode, one of said auxiliary electrodes being maintained at twice the potential of the other auxiliary electrode plus a constant.

10. The method of effecting distortionless modulation of carrier frequency oscillations by means of a thermionic tube having a control grid electrode, a cathode electrode and a plurality of auxiliary electrodes which includes the steps of, impressing the carrier frequency oscillations on said control grid electrode, and applying the modulating potentials to a pair of said auxiliary electrodes while maintaining the same at different direct current potentials with respect to said cathode, one of said auxiliary electrodes being maintained at twice the potential of the other auxiliary electrode plus a constant.

11. Signaling means comprising an electron discharge tube having input and output electrodes including a cathode, high frequency circuits connected with said input electrodes and with said output electrodes, means for tuning at least one of said circuits to resonance at high frequency, a plurality of auxiliary electrodes interposed between said input and output electrodes, a source of modulating potentials, and separate circuits applying modulating potentials from said source substantially in phase to said auxiliary electrodes.

12. In signaling means, a source of carrier frequency oscillations, an electron discharge tube having input and output electrodes including an anode and a cathode, means for energizing said input electrodes by oscillations from said source, an output circuit connected with said output electrodes, a plurality of auxiliary electrodes interposed between said input and output electrodes, a source of modulating potentials, and

separate circuits each including biasing means of different direct current value coupling said source of modulating potentials to said auxiliary electrodes and cathode and to said anode and cathode.

13. A signaling device comprising a thermionic tube having a cathode and having an input electrode coupled with a source of carrier frequency oscillations and an output electrode coupled with an output circuit, a plurality of auxiliary elec-i trodes interposed between said input and output electrodes, a source of modulating potentials, separate circuits for coupling said source of modulating potentials in phase to said auxiliary electrodes, and means for maintaining said auxiliary electrodes at different direct current potentials with respect to said cathode.

14. A transmitter comprising, a thermionic tube having a cathode and having an input electrode coupled with a source of carrier frequency oscillations and an output electrode connected with a load circuit, and means for producing distortionless modulation of said carrier frequency comprising, a plurality of auxiliary electrodes interposed between said input and said output electrodes, sources of potential for maintaining said auxiliary electrodes at different direct current potentials with respect to said cathode, and means for applying modulating potentials to each of said auxiliary electrodes.

15. A transmitter comprising, a thermionic I tube having a cathode and having an input electrode coupled with a source of carrier frequency oscillations and an output electrode connected with a load circuit, and means for producing distortionless modulation of said carrier frequency comprising, a plurality of auxiliary electrodes interposed between said input and said output electrodes, circuits including a source of potential for maintaining said auxiliary electrodes at different direct current potentials with respect to said cathode, and means for applying modulating potentials in phase to each of said circuits.

16. A signaling device comprising, a thermionic tube having anode, cathode and control grid electrodes, means for applying carrier frequency oscillations to said control grid and cathode, a load circuit connected between said anode and said cathode, and means for effecting distortionless modulation of the carrier frequency oscillations impressed on said tube comprising, a plurality of auxiliary electrodes interposed between said control grid and anode, circuits for maintaining said auxiliary electrode at different direct current potentials with respect to said cathode, a source of modulating potentials, and a coupling between said source of modulating potentials and said last named circuits and with said output circuit.

1'7. A signaling device comprising, a thermionic tube having anode, cathode and control grid electrodes, means for applying carrier frequency oscillations to said control grid and cathode, a load circuit connected between said anode and said cathode, and means for effecting distortionless modulation of the carrier frequency oscillations impressed on said tube comprising, a plurality of auxiliary electrodes interposed between said control grid and anode, circuits for maintaining said auxiliary electrodes at different direct current potentials with respect to said cathode, a source of modulating potentials, and a coupling between said source of modulating potentials and said last named circuits for impressing modulating potentials therein in phase and with said output circuit for impressing modulating potentials therein in phase opposition as compared to the phase of the potentials impressed on the prior mentioned circuits.

18. The method of signaling by means of a thermionic tube having a cathode electrode, anode electrode, and a plurality of auxiliary electrodes which include the steps of, impressing oscillations on one of said auxiliary electrodes, applying oscillations of different frequency in phase to a pair of said auxiliary electrodes and opposing changes in the potential of said anode electrode caused by said aforesaid steps.

HAROLD OLAF PETERSON. MARTIN KATZIN. 

